280 BOTANY PART 



sheath of the Banana which shows an elongation of I'l mm., and a 

 Bamboo shoot, with an increase in length of 0'75 mm. per minute ; a 

 strong shoot of Cucurbita grows 0*1 mm. per minute, the hyphae of 

 Botrytis grow 0'034 mm., while most other plants, even under favour- 

 able circumstances, attain but a small rate of elongation (0'005 mm. 

 and less per minute). 



The rate of growth of an organ never remains uniform ; even 

 under constant external conditions it gradually increases from very 

 small values to a maximum and then decreases to zero. This pheno- 

 menon is known as " the grand period of growth." An example will 

 illustrate its course. 



For the first internode of the stem of the Lupine, growing in the dark at a 

 constant temperature, the daily growth observed, measured in tenths of a milli- 

 metre, was : 



8, 9, 11, 12, 35, 43, 41, 50, 51, 52, 65, 54, 43, 37, 28, 18, 6, 2, o. 



The grand period is not always so regular as in this example ; 

 frequently deviations due to abrupt changes in the growth are 

 apparent. 



Distribution of Growth ( 6S ). As a rule any part of a plant is 

 not growing throughout its whole extent but consists of both fully- 

 grown and still growing portions. The latter also are not elongating 

 uniformly but are composed of zones, passing gradually into one 

 another, in which the rates of growth differ. The length and position 

 of the growing zones is not the same in different organs. The grow- 

 ing zone is longer in aerial roots and in extreme cases may amount to 

 1 m. In roots it is situated at the tip and occupies a length of 5 

 to 10 mm. The behaviour of stems varies. Those without sharply- 

 defined nodes have a single zone of growth of considerable length 

 (frequently extending to *5 m.). They thus resemble the aerial roots. 

 In many shoots, especially those divided into nodes and internodes, 

 there are a number of zones of growth separated by fully -grown zones. 

 This is termed intercalary growth and is beautifully shown, for 

 example, in the haulms of grasses, where a growing zone is found 

 at the base of each internode. At the bases of many leaves also, 

 especially of Monocotyledons, an intercalary growing zone is found. 



The distribution of growth in any member of the plant is ascertained by 

 periodically measuring the distance between certain natural or artificial marks. 



Thus, for example, the tip of the root in Fig. 256 7 is marked with lines of 

 india-ink at intervals of 1 mm. The marks start from the growing point of the 

 root (0) just behind the root-cap. Twenty-two hours later the marks had been 

 separated from one another as is shown in Fig. 256 //. The elongation has been 

 unequal in the different zones ; at the upper and lower ends of the marked 

 region it diminishes and thus leads to the fully-grown region on the one hand 

 and the embryonal region at the tip on the other. Between these and nearer to 

 the apical end is a zone where the maximal growth has taken place. If the 

 growth of one transverse zone such as that between and 1 is followed on 



